This invention relates to the engraving of cylinders commonly used in the gravure printing process, and specifically to engraving apparatus of the general type disclosed, for example, in U.S. Pat. Nos. 2,881,246, 2,874,479, 3,272,918, and 4,013,829. The basic principle of electro-mechanical engraving of a gravure cylinder involves rotating a copper plated cylinder while actuating an electrically driven tool which cuts or engraves cells or lines into the copper surface. The engraved cylinder is normally used in a web type gravure printing press for printing paper, plastic, or metallic film material.
In the gravure printing process, the engraved cylinder is flooded with ink, and a doctor blade wipes off excess ink from the surface so that only the engraved cells contain ink which is transferred to the material being printed. To obtain a high quality print, it is necessary that the cells be very accurately placed or located on the cylinder surface, usually within one to two microns of the desired theoretical location. The depth of the engraved cells must also be accurately controlled since the depth determines the amount of ink transferred which, in turn, determines the shade of gray in a black-white print. In a color print, the amount of ink transferred to the paper or material is even more critical since three colors are mixed to produce various shades of all possible colors. A slight variation in the desired amount of ink effects not only the darkness of the color but, more importantly, the production of the desired color tone.
In addition to printing newspapers and magazines, the engraved cylinders may also be used for direct or indirect printing of cloth, applying glue, printing of packaging materials for products, and printing of wood grain patterns for making wall paneling, floor coverings and other surface covers. A preferred apparatus for engraving a gravure printing cylinder is disclosed in pending U.S. patent application Ser. No. 056,623, filed July 11, 1979, now U.S. Pat. No. 4,357,633.
In the engraving of a gravure cylinder, the image pattern or copy to be engraved is usually mounted on a copy cylinder, and the copy is optically scanned while the engraving is being performed. However, the copy may be scanned and the corresponding information stored in computer memory, processed, and later used to engrave a cylinder. The engraving machine may be an electro-mechanical engraver which uses a diamond stylus to engrave the cylinder, or the machine may incorporate electronic means such as electron beam or laser for forming the cells within the cylinder. In either machine, a series of cavities and/or lines are engraved into the cylinder surface. The cavities are adapted to carry ink which produces the image on the material being printed. The image may involve either very small images such as printing type requiring very small and well defined lines or pictures requiring very close control of different cylinders for different colored inks needed for close color matching or large images for printing items such as wallpaper.
In the making of engraved cylinders for printing woodgrains, wall paper, and other printed patterns, there are various problems which in the past have been overcome by the use of a considerable amount of hand work, both in making the copy and in rework of the engraved cylinders. One of the problems is the need for large copy. Typically, a copy will range in size, for example, up to 60 inches by 60 inches, or even larger. With large copies, even with photographically produced copies, it is almost impossible to obtain a copy which has uniform density from end to end. When the two ends are joined on the engraved cylinder, the discontinuity in the film density shows up across the joint. A straight line joint in the copy is easier to recognize by the eye than a joint which is broken up, even by a small amount.
In the past, the copy for patterns such as woodgrains have been made by handworking a joint in the middle of the copy so that the ends have a repeat of patterns. It is then possible, with proper care for film density, to make the engraved joint directly across the copy. Usually there is a sizeable area of repeat in which the engraved joint can be made. The making of the hand made joint in the copy is very costly. It is usually made in a zig-zag manner extending for 6 to 12 inches or more. The exact path of the zig-zag is chosen to follow the woodgrains or across areas of similar woodgrains. Balance for film density is necessary, and larger grains of wood must be blended to make an invisible joint. Elimination of this hand made joint is highly desirable.